Ultra-short optical pulse transverse second harmonic measurement system that suppresses back-reflected light

Abstract

The application relates to a kind of ultra-short light pulse transverse second harmonic devices for inhibiting retroreflected light, comprising polarization beam splitter arranged in the direction of the measured ultra-short pulse light beam, transmitting the first polarization component of the measured ultra-short pulse to form the first polarized light beam, reflecting the second polarization component of the measured ultra-short pulse to form the second polarized light beam;First wave plate, converting the first polarized light beam into first circularly polarized light of first rotation direction to form the first converted light beam;Second wave plate, converting the second polarized light beam into second circularly polarized light of second rotation direction to form the second converted light beam;First mirror group and second mirror group, respectively, make the first converted light beam and the second converted light beam vertically incident on the disordered nonlinear medium in opposite directions, and generate transverse second harmonic inside the disordered nonlinear medium.The ultra-short light pulse transverse second harmonic device for inhibiting retroreflected light described in the application can inhibit the retroreflected light introduced in the reverse collinear optical path at low cost, and prevent damage to the front-end system such as light source.

Description

Technical Field

[0001] This invention relates to the field of ultrafast optical measurement and characterization technology, and in particular to an ultrashort optical pulse transverse second harmonic measurement system that suppresses backlight. Background Technology

[0002] With the continuous iteration and upgrading of laser science and technology, ultrashort light pulses have gradually become a widely used light source, increasingly applied in various industries, and have become an indispensable tool. Common industrial applications include optical communication, laser cutting, and micromachining; their applications in scientific research are equally broad, such as ultrafast spectral analysis, observing the dynamic behavior of electrons, measuring the dynamic properties of complex DNA molecules, and monitoring chemical reactions. Therefore, the demands for ultrashort light pulses in various fields are becoming increasingly stringent. To ensure the precise application of these ultrashort light pulses in these fields, accurate monitoring of their parameters is crucial.

[0003] Traditional ultrashort optical pulse measurement systems utilize nonlinear crystals to generate sum-frequency second harmonics (SHMs) for measurement. However, this approach suffers from complexities such as phase matching conditions and temperature tuning, resulting in high instrument accuracy requirements and high operating costs. Some researchers have proposed using the transverse second harmonic effect to perform single-shot measurements of ultrashort pulse widths, overcoming the stringent phase matching requirements, temperature dependence, and narrow matching bandwidth of conventional nonlinear materials. However, existing methods generate high-intensity backlighting, posing a significant safety threat to front-end instruments such as femtosecond laser sources. Summary of the Invention

[0004] Based on this, the purpose of the present invention is to provide a transverse second harmonic measurement system for ultrashort optical pulses that suppresses backlighting. This system can achieve high-precision single-shot measurement of ultrashort optical pulses while completely suppressing backlighting introduced in the reverse collinear optical path, thus preventing damage to the front-end system such as the light source.

[0005] A transverse second harmonic device for suppressing back-reflected light from an ultrashort optical pulse includes a polarizing beam splitter disposed in the direction of the ultrashort pulse being measured. The first polarization component of the ultrashort pulse being measured is transmitted to form a first polarized beam, and the second polarization component of the ultrashort pulse being measured is reflected to form a second polarized beam; wherein the first polarization component and the second polarization component are orthogonal.

[0006] The first waveplate converts the first polarized beam into first spirally polarized light to form the first converted beam.

[0007] The second waveplate converts the second polarized beam into circularly polarized light with a second rotation direction to form a second converted beam; wherein the first rotation direction and the second rotation direction are opposite to each other;

[0008] The first set of reflecting mirrors changes the propagation direction of the first converted beam to form the first incident beam;

[0009] The second set of mirrors changes the propagation direction of the second converted beam to form a second incident beam; and

[0010] A disordered nonlinear medium, wherein the first incident beam and the second incident beam are incident perpendicularly to the surface of the disordered nonlinear medium; wherein the first incident beam and the second incident beam generate transverse second harmonics and corresponding transverse second harmonic autocorrelation signals within the disordered nonlinear medium.

[0011] Compared with existing ultrashort optical pulse transverse second harmonic wave (THF) devices, the present invention provides an ultrashort optical pulse THF device for suppressing backlight. By using a quarter-wave plate to generate two circularly polarized beams with opposite rotation directions, and using these two circularly polarized beams to excite the transverse second harmonic autocorrelation signal, backlight suppression in the optical path can be achieved at low cost, ensuring the safety of the front-end system in the optical path.

[0012] Furthermore, both the first and second waveplates are quarter-wave plates, which convert the two linearly polarized beams from the polarizing beam splitter into circularly polarized beams, and restore the circularly polarized beams that return after passing through the disordered nonlinear medium to the original polarized linearly polarized beams.

[0013] Furthermore, the first reflector group includes a first reflector and a second reflector, with the first reflector disposed between the second reflector and the first waveplate; the second reflector group includes a third reflector and a fourth reflector, with the third reflector disposed between the fourth reflector and the second waveplate.

[0014] Furthermore, it also includes at least one translation stage, which is disposed at the first reflector and the second reflector. The translation stage is controlled to move the first reflector and the second reflector to change the optical path of the first converted beam, thereby causing the overlapping position of the first incident beam and the second incident beam inside the disordered nonlinear medium to move along the collinear direction of the first incident beam and the second incident beam.

[0015] Furthermore, it also includes at least one translation stage, which is disposed at the first reflector and the third reflector. The translation stage is controlled to move the first reflector and the third reflector so that the overlapping position of the first incident beam and the second incident beam inside the disordered nonlinear medium moves in a direction perpendicular to the collinearity of the first incident beam and the second incident beam.

[0016] Furthermore, it also includes a first aperture stop and a second aperture stop. The first aperture stop is disposed between the disordered nonlinear medium and the first mirror group to change the degree of energy attenuation of the first incident beam. The second aperture stop is disposed between the disordered nonlinear medium and the second mirror group to change the degree of energy attenuation of the second incident beam.

[0017] Furthermore, it also includes a focusing lens, which is disposed between the laser generating unit and the polarizing beam splitter to focus the initial beam so that the initial beam is focused inside the disordered nonlinear medium.

[0018] Furthermore, the device includes the aforementioned device and measurement unit for suppressing the retroreflected light in an ultrashort optical pulse. The measurement unit receives the transverse second harmonic and the corresponding transverse second harmonic autocorrelation signal, measures the spatial intensity distribution of the transverse second harmonic autocorrelation signal, and performs image analysis to obtain parameter information of the initial ultrashort optical pulse.

[0019] Furthermore, the measurement unit includes an imaging magnification system, a CCD, and a processor; the imaging magnification system collects and amplifies the transverse second harmonic and corresponding autocorrelation signals generated inside the disordered nonlinear medium; the CCD records the amplified signal image; the processor is connected to the CCD, displays the signal image in real time on the processor window, and analyzes and obtains the parameter information of the initial ultrashort optical pulse.

[0020] Furthermore, the parameter information of the initial ultrashort optical pulse is obtained by processing and analysis in the following manner: a first acquired image is recorded by CCD when the autocorrelation signal can be observed, the first acquired image containing the autocorrelation signal and the background signal; the translation stage is adjusted to move the autocorrelation signal out of the CCD lens, and a second acquired image is recorded at this time, the second acquired image containing the background signal; the first acquired image and the second acquired image are subtracted by a processor to obtain a filtered image, the filtered image containing the corresponding autocorrelation signal; data software is used in the processor to identify and process the filtered image to obtain a clear transverse second harmonic autocorrelation signal envelope; the transverse second harmonic autocorrelation signal envelope is analyzed accordingly to obtain the parameter information of the initial ultrashort optical pulse.

[0021] To better understand and implement this invention, the following detailed description is provided in conjunction with the accompanying drawings. Attached Figure Description

[0022] Figure 1 This is a structural diagram of the transverse second harmonic measurement system for suppressing backlighting using ultrashort optical pulses according to the present invention.

[0023] Figure 2This is a schematic diagram of the collision of two anti-rotating circularly polarized beams in the transverse second harmonic measurement system of the ultrashort optical pulse for suppressing backlighting of the present invention.

[0024] Figure 3 This is a graph showing the transverse second harmonic signal curves corresponding to the change in the spin angle between the first and second incident beams at the point of overlap within a disordered nonlinear medium in the transverse second harmonic measurement system of the ultrashort optical pulse for suppressing backlight of the present invention.

[0025] Figure 4 The image shows the transverse second harmonic image generated by the collision of a first incident beam and a second incident beam in a disordered nonlinear medium in the ultrashort optical pulse transverse second harmonic measurement system for suppressing backlighting of the present invention.

[0026] Figure 5 To Figure 4 The autocorrelation signal image after filtering out the second harmonic signal background.

[0027] Figure 6 According to Figure 5 The autocorrelation signal fitting diagram extracted from the transverse second harmonic image after filtering out the harmonic background. Detailed Implementation

[0028] The applicant carefully analyzed existing ultrashort optical pulse measurement methods utilizing the transverse second harmonic effect and found that the safety threat to front-end instruments such as femtosecond lasers in current transverse second harmonic measurement methods is due to the presence of high-intensity backlight in the colliding optical path. Therefore, the applicant believes that it is necessary to suppress backlight in the system. However, the traditional method of suppressing backlight is to use an optical isolator adapted to ultrafast lasers, which is relatively expensive. Therefore, the applicant believes that it is necessary to design a transverse second harmonic measurement system that can suppress backlight in the optical path at a relatively low cost. This system uses a quarter-wave plate to generate two circularly polarized beams with opposite directions of rotation and uses these two circularly polarized beams to excite the transverse second harmonic autocorrelation signal. The applicant has rigorously proven its feasibility in theory and actually tested the results of the transverse second harmonic autocorrelation signal excited by two opposing circularly polarized beams to prove that the design achieves both the backlight suppression effect and ensures the accuracy of the ultrafast laser pulse width measurement results.

[0029] Example 1

[0030] Based on this, please refer to Figure 1 The ultrashort optical pulse transverse second harmonic measurement system for suppressing backlight of the present invention includes a laser generating unit 1, a focusing lens 2, a polarizing beam splitter 3, a first waveplate 41, a second waveplate 42, a first reflecting mirror group, a second reflecting mirror group, a first aperture stop 61, a second aperture stop 62, a disordered nonlinear medium 7, and a measurement unit 8.

[0031] The laser generating unit 1 continuously emits an initial beam containing the ultrashort pulse to be measured. The initial beam is linearly polarized light, and its wavelength is preferably 800 nm. It is understood that the ultrashort pulse to be measured can have various forms, including a single ultrashort laser pulse or a sequence of ultrashort laser pulses.

[0032] The focusing lens 2 is a common convex lens, which is set in the propagation direction of the initial beam to focus the initial beam into a focused beam, so that the initial beam is focused inside the disordered nonlinear medium 7 after passing through a series of devices.

[0033] The polarizing beam splitter 3 can separate linearly polarized light into mutually orthogonal p-polarized and s-polarized components, positioned along the propagation direction of the focused beam. Specifically, the p-polarized component in the transmitted focused beam forms the first polarized beam, and the s-polarized component in the reflected focused beam forms the second polarized beam. It can be understood that the separation directions of the p-polarized and s-polarized components of the incident beam can be interchanged, meaning the s-polarized component in the transmitted focused beam forms the first polarized beam, and the p-polarized component in the reflected focused beam forms the second polarized beam. In this case, other polarization-matching settings in the system are also simultaneously interchanged.

[0034] Both the first waveplate 41 and the second waveplate 42 are quarter-wave plates, capable of converting incident linearly polarized light into circularly polarized light. Specifically, the first waveplate 41 is positioned along the propagation direction of the first polarized beam, converting it into left-handed circularly polarized light to form the first converted beam. The second waveplate 42 is positioned along the propagation direction of the second polarized beam, converting it into right-handed circularly polarized light to form the second converted beam. It is understood that the first converted beam is not limited to left-handed circularly polarized light. When the linear polarization direction of the initial beam is changed or the separation law of the polarizing beam splitter 3 is altered, the first converted beam can also be right-handed circularly polarized light, in which case the second converted beam is left-handed circularly polarized light, provided that the first and second converted beams are oppositely polarized. It is understood that the second converted beam, after passing through the disordered nonlinear medium 7, will illuminate the first waveplate 41, which can then convert it back into linearly polarized light, thus avoiding the influence of backlighting in the optical path.

[0035] The first reflector group is positioned along the propagation direction of the first converted beam, reflecting the first converted beam to change its propagation direction and form a first incident beam, thereby achieving perpendicular incidence. The first reflector group includes a first reflector 511 and a second reflector 512, with the first reflector 511 positioned between the second reflector 512 and the first waveplate 41.

[0036] The second reflector group is positioned along the propagation direction of the second converted beam, reflecting the second converted beam to change its propagation direction and form a second incident beam, thus allowing it to be incident perpendicularly. The second reflector group includes a third reflector 521 and a fourth reflector 522, with the third reflector 521 positioned between the fourth reflector 522 and the second waveplate 42.

[0037] The first aperture stop 61 is positioned between the first reflector group and the first aperture stop 61. It can change its aperture size, thereby changing the degree of energy attenuation of the first incident beam and preventing the energy of the first incident beam from being too high and damaging the disordered nonlinear medium 7.

[0038] The second aperture stop 62 is positioned between the second reflector group and the second aperture stop. It can change its own aperture size, thereby changing the degree of energy attenuation of the second incident beam and preventing the second incident beam from being too high in energy and damaging the disordered nonlinear medium 7.

[0039] The disordered nonlinear medium 7 is a nanocrystalline glass, a composite material formed by embedding nonlinear nanocrystals within the glass. This material can generate transverse second harmonics under certain conditions. Specifically, the first incident beam and the second incident beam propagate perpendicularly to the surface of the disordered nonlinear medium 7 along opposite propagation directions, and the first and second incident beams are strictly collinear within the disordered nonlinear medium 7. Transverse second harmonics and their autocorrelation signals are generated within the disordered nonlinear medium 7, thereby forming a probe beam perpendicular to the propagation direction of the first incident beam. It should be noted that the probe beam generated by the transverse second harmonic effect of the nanocrystalline glass exists across the entire cross-section perpendicular to the propagation direction of the first incident beam. Furthermore, it is understood that the use of nanocrystalline glass as the disordered nonlinear medium 7 in the ultrashort optical pulse transverse second harmonic measurement system for suppressing backlighting in this invention takes into account the cost and convenience of nanocrystalline glass, and is not limited to using only nanocrystalline glass. Other materials capable of generating transverse second harmonics can also meet the requirements, and those skilled in the art can make corresponding adjustments based on actual conditions.

[0040] The measurement unit 8 is positioned along the propagation direction of the probe beam, receives the probe beam, measures the spatial intensity distribution of the transverse second harmonic autocorrelation signal within it, and performs image analysis to obtain parameter information of the initial ultrashort optical pulse. Please refer to... Figure 2 When two anti-polarized circularly polarized beams of the same frequency propagate towards each other, observing along the propagation direction of one of the beams, it can be seen that at a certain point where the two beams coincide in space, their rotating electric vectors will always maintain a fixed angle. Similarly, at the point where the first and second incident beams coincide within the disordered nonlinear medium 7, there exists a polarization angle that varies linearly with spatial position, and this polarization angle is determined by the meeting point of the two beams; please refer to [link to relevant documentation]. Figure 3 , Figure 3 The diagram illustrates the spatial intensity envelope of the transverse second harmonic autocorrelation signal and the relationship between the spatial intensity and the rotation angle in the amplified curve: when the rotation angle is 0 / 180°, the electric vectors of the first and second incident beams will generate the peak value of the transverse second harmonic autocorrelation signal at this position inside the disordered nonlinear medium 7. As the spatial position changes, the relative angle between the two photoelectric vectors also changes rapidly, causing the intensity of the excited autocorrelation signal to oscillate rapidly in sync. However, the frequency of its autocorrelation signal intensity change is comparable to the spatial frequency of the incident light, and the resulting modulation effect will be averaged by the detector and will not affect the measurement and accuracy of the transverse second harmonic autocorrelation signal.

[0041] Example 2

[0042] Compared to Embodiment 1, this second embodiment adds only one translation stage (not shown in the figure). The translation stage allows the first or second reflector group to be moved, shifting the overlap position of the first and second incident beams within the disordered nonlinear medium 7, thus achieving fine adjustment of the excitation position of the disordered nonlinear medium 7. Specifically, translation stages are provided at the first reflector 511 and the second reflector 512. Controlling these stages moves the first and second reflectors 511 and 512, changing the optical path of the first converted beam. This causes the overlap position of the first and second incident beams within the disordered nonlinear medium 7 to move along the collinear direction of the first and second incident beams. Additionally, a translation stage can be provided at the third reflector 521. Controlling this stage moves the first and third reflectors 511 and 521, causing the overlap position of the first and second incident beams within the disordered nonlinear medium 7 to move along the perpendicular direction of their collinearity. The remaining structure is identical to that in Embodiment 1 and will not be described again here.

[0043] To further ensure the accuracy of the transverse second harmonic measurement system of the present invention for suppressing backlight ultrashort optical pulses, the applicant used measurement unit 8 to acquire the optical image after the collision of the first incident beam and the second incident beam, and analyzed the transverse second harmonic fitting to obtain the transverse second harmonic autocorrelation signal function, thereby confirming the accuracy of the measurement.

[0044] Specifically, the measurement unit 8 includes an imaging magnification system, a CCD, and a processor. The imaging magnification system collects and amplifies the transverse second harmonic and corresponding autocorrelation signals generated inside the disordered nonlinear medium 7. The amplified signal image is recorded by the CCD located at the image plane. The processor is connected to the CCD and displays the signal image in real time on the processor window. The processor analyzes the signal image to obtain the parameter information of the initial ultrashort optical pulse.

[0045] Please see Figure 4 , Figure 4 The optical image after the collision of the first and second incident beams is shown. The processor records the first acquired image, where the autocorrelation signal is observable, as "autocorrelation signal + background." It can be seen that the brightness is most pronounced at the location where the transverse second harmonic autocorrelation signal is generated. Subsequently, the translation stage is adjusted to move the autocorrelation signal outside the CCD lens, and the second acquired image at this point is recorded as "background." Please refer to [link to relevant documentation]. Figure 5 , Figure 5 The image shows the transverse second harmonic and corresponding autocorrelation signals in the optical image after the collision of the first and second incident beams. The processor subtracts the first and second acquired images, canceling out the background signal and leaving only the original image. Figure 5 The filtered image shown contains the corresponding autocorrelation signal. Data software in the processor identifies and processes the filtered image to obtain a clear transverse second harmonic autocorrelation signal envelope; the transverse second harmonic autocorrelation signal envelope is then analyzed to obtain the parameter information of the initial ultrashort optical pulse.

[0046] Please see Figure 6 , Figure 6 The transverse second harmonic function obtained by fitting a transverse second harmonic image with the background harmonics filtered out is shown. It can be seen that the fitted transverse second harmonic peak is single and distinct, proving that the ultrashort optical pulse transverse second harmonic measurement system for suppressing backlight of this invention can guarantee the accuracy of measuring the transverse second harmonic autocorrelation signal.

[0047] The ultrashort optical pulse transverse second harmonic measurement system of the present invention suppresses backlight by using a quarter-wave plate to generate two circularly polarized beams with opposite rotation directions. These two circularly polarized beams are then used to excite the transverse second harmonic autocorrelation signal, which is analyzed to obtain the parameter information of the initial ultrashort optical pulse. This system achieves backlight suppression at low cost while ensuring the accuracy of ultrafast laser pulse width measurement results.

[0048] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and the present invention also intends to include these modifications and variations.

Claims

1. A device for suppressing the transverse second harmonic of an ultrashort optical pulse, characterized in that: Including those set in the direction of the measured ultrashort pulse beam: A polarizing beam splitter transmits the first polarization component of the measured ultrashort pulse to form a first polarized beam and reflects the second polarization component of the measured ultrashort pulse to form a second polarized beam; wherein the first polarization component and the second polarization component are orthogonal. The first waveplate converts the first polarized beam into first spirally polarized light to form the first converted beam. The second waveplate converts the second polarized beam into circularly polarized light with a second rotation direction to form a second converted beam; wherein the first rotation direction and the second rotation direction are opposite to each other; The first set of reflecting mirrors changes the propagation direction of the first converted beam to form the first incident beam; The second set of mirrors changes the propagation direction of the second converted beam to form a second incident beam; A disordered nonlinear medium, wherein the first incident beam and the second incident beam are incident perpendicularly to the surface of the disordered nonlinear medium; wherein the first incident beam and the second incident beam generate transverse second harmonics and corresponding transverse second harmonic autocorrelation signals within the disordered nonlinear medium.

2. The ultrashort optical pulse transverse second harmonic suppression device for suppressing backlighting according to claim 1, characterized in that: The first waveplate and the second waveplate are both quarter-wave plates, which convert the two linearly polarized beams from the polarizing beam splitter into circularly polarized beams, and restore the circularly polarized beams that return after passing through the disordered nonlinear medium to the original polarized linearly polarized beams.

3. The ultrashort optical pulse transverse second harmonic suppression device for retroreflected light according to claim 2, characterized in that: The first reflector group includes a first reflector and a second reflector, with the first reflector disposed between the second reflector and a first waveplate; the second reflector group includes a third reflector and a fourth reflector, with the third reflector disposed between the fourth reflector and the second waveplate.

4. The ultrashort optical pulse transverse second harmonic suppression device for retroreflected light according to claim 3, characterized in that: It also includes at least one translation stage, which is disposed at the first reflector and the second reflector. The translation stage is controlled to move the first reflector and the second reflector to change the optical path of the first converted beam, so that the overlapping position of the first incident beam and the second incident beam inside the disordered nonlinear medium moves along the collinear direction of the first incident beam and the second incident beam.

5. The ultrashort optical pulse transverse second harmonic suppression device for retroreflected light according to claim 3, characterized in that: It also includes at least one translation stage, which is disposed at the first reflector and the third reflector. The translation stage is controlled to move the first reflector and the third reflector so that the overlapping position of the first incident beam and the second incident beam inside the disordered nonlinear medium moves in a direction perpendicular to the collinearity of the first incident beam and the second incident beam.

6. The ultrashort optical pulse transverse second harmonic suppression device for suppressing backlight according to any one of claims 3 to 5, characterized in that: It also includes a first aperture stop and a second aperture stop. The first aperture stop is disposed between the disordered nonlinear medium and the first mirror group to change the degree of energy attenuation of the first incident beam. The second aperture stop is disposed between the disordered nonlinear medium and the second mirror group to change the degree of energy attenuation of the second incident beam.

7. The ultrashort optical pulse transverse second harmonic suppression device for retroreflected light according to claim 6, characterized in that: It also includes a focusing lens, which is disposed between the laser generating unit and the polarizing beam splitter to focus the initial beam so that the initial beam is focused inside the disordered nonlinear medium.

8. A transverse second harmonic measurement system for suppressing backlighting using ultrashort optical pulses, characterized in that: The device for suppressing retroreflected light in an ultrashort optical pulse with transverse second harmonic distortion (THD) as described in claims 1-7, and a measurement unit, wherein the measurement unit receives the THD and the corresponding THD autocorrelation signal, measures the spatial intensity distribution of the THD autocorrelation signal and performs image analysis to obtain parameter information of the initial ultrashort optical pulse.

9. The ultrashort optical pulse transverse second harmonic measurement system for suppressing backlight according to claim 8, characterized in that: The measurement unit includes an imaging magnification system, a CCD, and a processor; the imaging magnification system collects and amplifies the transverse second harmonics and corresponding autocorrelation signals generated inside the disordered nonlinear medium; the CCD records the amplified signal image; the processor is connected to the CCD, displays the signal image in real time on the processor window, and analyzes the parameter information of the initial ultrashort optical pulse.

10. The ultrashort optical pulse transverse second harmonic measurement system for suppressing backlight according to claim 9, characterized in that: The parameter information of the initial ultrashort optical pulse is obtained by processing and analyzing it as follows: A first acquired image is recorded using a CCD when the autocorrelation signal can be observed. The first acquired image contains the autocorrelation signal and the background signal. The translation stage is adjusted so that the autocorrelation signal moves out of the CCD lens, and a second acquired image is recorded at this time. The second acquired image contains the background signal. The processor subtracts the first acquired image from the second acquired image to obtain a filtered image, which contains the corresponding autocorrelation signal. Data software is used in the processor to identify and process the filtered image to obtain a clear transverse second harmonic autocorrelation signal envelope. The transverse second harmonic autocorrelation signal envelope is analyzed accordingly to obtain the parameter information of the initial ultrashort optical pulse.

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